For several decades microelectronics progressed in decreasing size while simultaneously increasing complexity. Mechanical structures can now be fabricated to comparable dimensions, and can be highly integrated with the electronics into microelectromechanical systems (MEMS).
MEMS can now be realized as sensors, actuators or mechanical structures. Products using MEMS are used in such life-saving devices as airbag accelerometers and disposable blood-pressure transducers to monitor the heart rate. MEMS devices have become more complex, and have incorporated an increasing number of features, progressing from components to electronic systems. The trend is measured in terms of the level of integration of MEMS mechanic and electronics.
The motion of the micro mechanical elements in micro-sensors is generally proportional to a particular physical measurement of interest, such as acceleration or partial pressure. Therefore it is important to sense this motion, in-order to reproduce the physical measure. In devices of more general application, the motion sensing may be used for feedback control, for example, to restore the micro-machined device to its original physical location. In high-grade sensors, such as inertial grade gyroscopes and accelerometers, as well as high-grade microphones, the sensing of motion in the sub-picometer (pm) range is required. Moreover, the motion sensing apparatus, itself, should be small and integrative, in-order to function effectively in the dimensions of the micro-machined devices.
Although the same laws of physics apply in three-dimensional structures, in the micron and sub-micron scales, nonlinear effects that are generally negligible in the macro-environment become more significance. Generally, micro-machined proof-masses can move in six degrees of freedom: three along linear axes: x,y and z; and three angles of rotation about these axes.
The main factor that limits the minimum detectable displacement is the noise sources that sensing systems are susceptible to. Spurious noise sources include mechanical noise, electrical noise, light noise, etc. Noise sources can be either “white” noise, that is frequency independent, or frequency dependent, for example “1/f” noise that is inversely dependent on frequency.
Many motion sensing techniques are known in the art of micro-machined sensors and devices, as described in the following background references:    [1] J. A. Plaza, H. Chen, J. Esteve and E. Lora-Tamayo, “New bulk accelerometer for triaxial detection”, Sensors and Actuators A: Physical, Vol. 66, 1998, pp. 105–108.    [2] R. Voss, K. Bauer, W. Ficker, T. Gleissner, W. Kupke, M. Rose, S. Sassen, J. Schalk, H. Seidel and E. Stenzel, “Silicon angular rate sensor for automotive applications with piezoelectric drive and piezoresistive read-out”, Proc. of Transducers'97, Chicago, 16–19 Jun. 1997, pp. 879–882.    [3] Y. Nemirovsky, A. Nemirovsky, P. Murlat and N. Setter, “Design of a novel thin-film piezoelectric accelerometer”, Sensors and Actuators A: Physical, Vol. 56, 1996, pp. 239–249.    [4] B. E. Boser and R. T. Howe, “Surface micromachined accelerometers”, J. of Solid-State Circuits, Vol. 31, No. 3, March 1996, pp. 366–375.    [5] M. Weinberg, J. Connelly, A. Kourepenis and D. Sargent, “Microelectromechanical instrument and systems development at the Charles Stark Draper Laboratory, INC.”, Proceedings of the IEEE Digital Avionics Systems Conference, 1997, pp. 8.5–33–8.5–40.    [6] H. K. Rocksatd, T. W. Kenny, J. K. Reynolds, W. J. Kaiser and T. B. Gabrielson, “A miniature high-sensitive broad-band accelerometer based on electron tunneling transducers”, Sensors and Actuators A: Physical, Vol. 43, 1994, pp. 107–114.    [7] R. L. Kubena, D. J. Vickers-Kirby, R. J. Joyce and Frederick P. Stratton, “A new tunneling based sensor for inertial rotation rate measurements”, JMEMS, Vol. 8, no. 4, December 1999, pp. 439–447.    [8] U. A. Dauderstadt, P. H. S. de Vries, R. Hiratsuka and P. M. Sarro, “Silicon accelerometer based on thermopiles”, Sensors and Actuators A: Physical, Vol. 46–47, 1995, pp. 201–204.    [9] O. Degani, “Investigation of Microelectromechanical Systems employing Modulated Integrative Differential Optical Sensing”, M. Sc. Thesis, Supervised by Y. Nemirovsky, Technion, 1999.    [10] T. Storgaard-Larsen, S. Bouwstra and O. Leistiko, “Opto-mechanical accelerometer based on strain sensing by bragg grating in a planar waveguide”, Sensors and Actuators A: Physical, Vol. 52, 1996, pp. 25–32.    [11] G. Schopfer, W. Elflein, M. de Labachelerie, H. Porte and S. Ballandras, “Lateral optical accelerometer micromachined in (100) silicon with remote readout based on coherence modulation”, Sensors and Actuators A: Physical, Vol. 68, 1998, pp. 344–349.    [12] T. B. Gabrielson, “Mechanical-Thermal Noise in Micromachined Acoustic and Vibration Sensors”, IEEE Trans. On Elec. Dev., Vol. 40, No. 5, May 1993.    [13] M. Bao, H. Yang, H. Yin and S. Shen, “Effects of electrostatic forces generated by the driving signal on capacitive sensing devices”, Sensors and Actuators A: Physical, Vol. 84, 2000, pp. 213–219.    [14] S. P. Timoshenko, I. N. Goodier, Theory of Elasticity, McGraw-Hill, New-York, 1970.    [15] D. D. Lynch, “Coriolis vibratory gyros”, Proc. of GYRO technology symposium, Stuttgart, Germany, 15–16 Sep. 1998, pp. 1.0–1.14.
As described in the above references, the known motion sensing techniques using micro-machined sensors and devices include: piezoresistive [1,2], piezoelectric [3], capacitive [4,5], tunneling [6,7], thermal [8] and optical [9-11] sensing. Few methods have shown the capability for sensing motions in the sub-picometer range.
The most sensitive method, so far, is based on the tunneling effect between a sharp tip and a facing electrode. It has been shown theoretically [12] that the Noise Equivalent Displacement (NED) of this method, at the medium frequencies range, is in the order of 10−2–10−3[pm/√Hertz] (10−4–10−5[_/√Hertz]) (Hz). Nevertheless, the tunneling effect suffers from inherent “1/f” noise, up to the range of a few Kilohertz (KHz), and its apparatus is quite difficult to realize. Moreover, due to the close proximity required between the tip and the facing electrode, of the order of a few _ (Angstrom units), the sensing is adversely affected by a relatively high damping coefficient and thus a high thermal-mechanical noise.
Another technique, which is the one most commonly used in micro-machined devices, and which have shown to be sufficiently sensitive, is capacitive sensing. This method is rather simple to realize and is highly integrable. Capacitive sensing does not suffer from inherent noise sources. The noise of the capacitive transducer is mainly contributed by the electronic readout circuits or by thermal-mechanical noise. Therefore, by proper design of the electronic readout, capacitive sensing is limited only by thermal-mechanical noise, and theoretically a NED of 10−2[pm/√Hz] (10−4[_/√Hz]) can be achieved.
Capacitive sensing, on the other hand, suffers from possible cross talk with the electronic readout signals, which may exert a parasitic force on the device [13]. Moreover, without proper shielding it may suffer from Electromagnetic Interference (EMI). Due to the close proximity between the capacitor plates, which is required to achieve high sensitivity, the capacitive transducer is subjected to a rather high damping coefficient, which results in a higher thermal-mechanical noise. To lower the damping, vacuum encapsulation is usually required for high-grade sensors.
More recently, a piezoresistive configuration with a high degree of symmetry was reported to yield a NED in the order of a few [pm/√Hz], which is not quite low enough, but is rather near to the order required [14]. The piezoresistive method is quite easy to realize, and integrate, and was one of the first sensing methods to be used in micro-sensors. Moreover, it does not require close proximity between surfaces, and thus, benefits from a low damping coefficient and low thermal-mechanical noise. Nevertheless, it suffers from a high temperature coefficient, which limits the micro-sensor's performance.
Certain aspects of this problem are addressed in the above-referenced co-pending Israeli Patent Application No. 122,947, which is assigned to the assignees of the present patent application and discloses a viable system to sense sub-angstrom displacements. As with the capacitive method, MIDOS [9] does not suffer from inherent 1/f noise. As an improvement over capacitive methods, MIDOS is neither susceptible to EMI nor to cross talk from readout circuits. The integration of the motion sensing elements, the photodiodes, and the readout electronics on the same chip also reduces the system noise. Moreover, the sensing is based on in-plane motion, and close proximity is not required between the mechanical elements. Thus, the damping coefficient is lower, and the thermal-mechanical noise, also, is at lower vacuum levels than those required for the capacitive transducer. Nevertheless, the NED of the MIDOS technique is still far from achieving the demands of high-grade sensors.
No other sensing technique has as yet shown a NED lower than 1[pm/√Hz].
Thus there is a need for a micro-machined sensor capable of detecting motion in the sub-picometer range, and without the drawbacks of prior art devices.